K.
My now 3 1/2 year old twins were breastfed until they were 2 1/2. There were points along the way where tandem feeding was easier and tougher, depending on their development. Is it possible that you're just needing to try a new hold in order to still tandem feed? I kind of remember having some challenges when I needed to switch from a football hold to a different hold. I ended up finding a way to modify the cross cradle hold in a way that allowed them to nurse comfortably together (they kinda looked like a heart shape). I will say that I didn't have the same problem you describe of leaking while single nursing, so that wasn't an issue for us.
Good luck!